1. Field of the Invention
The present invention relates to an X-ray CT apparatus that controls a collimator for restricting the irradiation field of X-rays (or X-ray beam).
2. Description of the Related Art
X-ray computed tomography apparatuses are known by which X-rays are irradiated at a patient and image data is reconstructed from the permeation data. An X-ray computed tomography apparatus for multi-slicing makes it possible to collect data regarding a plurality of slices from different positions all together at a time by using an X-ray detector in which detecting elements (e.g., an assembly of a scintillator and photo diode) for detecting X-rays are arranged in a row. The combined use of multi-slice scanning (also referred to as cone-beam scanning) and helical scanning makes it possible to collect data of an extremely wide scanning range within a short time, so propagation in the future is anticipated.
One important objective in the case of the combined use of multi-slice scanning and helical scanning is the reduction of exposure to radiation dose. For example, there is technology in which a scanning range is set so as to include the region of a subjected organ on a scanogram and the opening of a collimator is set in accordance with the scanning range so that the scanning is restricted to the subjected organ within the patient (e.g., Japanese Unexamined Patent Application Publication No. 2002-17716 and Japanese Unexamined Patent Application Publication No. H10-248835). However, in reality, a part of the targeted organ is left out from the scanning range, resulting in missing data. Therefore, a situation may occur in which rescanning becomes necessary.
In this respect, it is possible to reconstruct a scanned image even if the data is missing, but in reality, noise ends up being inserted into the portion of the reconstructed scanned image where the data was missing. In addition, when a doctor uses the scanned image as a reference for diagnosis, the area of the diagnosis such as cancer is extremely small. Therefore, it is difficult to distinguish the difference of the noise that has been inserted into the scanned image from a diseased site, so there is a risk of misdiagnosis. For this reason, it is necessary to reduce the amount of exposure to radiation without missing data.
Accordingly, technology has been proposed in the past for reducing the radiation exposure of a patient at the time of helical scanning by changing the opening degree (size of the irradiation field) of the opening of a collimator in accordance with the position on a body axis in the progressing course of the helical scanning (e.g., Japanese Unexamined Patent Application Publication No. 2006-51233).
However, with the conventional method of changing the opening degree of the opening of a collimator, only the coordinate of the direction of movement of the bed of the collimator opening is acquired, and the opening degree of the opening of the collimator is preliminarily determined with respect to said coordinate. Thus, it is difficult to say that the timing of data collection via a data collection system corresponds to the control of the opening of the collimator, the data collection system including an imaging part such as an X-ray irradiator or an X-ray detector. Therefore, correlation of the collected data and the opening degree of the collimator is unclear, and there is a risk of missing data or of subjecting a patient to excessive radiation exposure.
Furthermore, in recent years, technology referred to as variable speed helical scanning has been proposed, whereby scanning is performed by changing the speed of movement in the body axis direction and the helical pitch while collecting data of a designated scanning range. In this respect, the collimator is simply controlled according to the distance by which the bed is moved, so with the conventional method of collimator control, it has been difficult to control the opening of the collimator with favorable accuracy with respect to the changes of the speed of movement or the helical pitch in such variable speed helical scanning.